High data transfer rates are achieved in wireless communications. Difficulties must be overcome to successfully transfer data over a wireless band at high rates. Noise, e.g., intersymbol interference, fading, etc., degrades the signal seen by the antenna of a receiver. One technique to address this is the use of multiple antennas and Orthogonal Frequency Division Multiplexing (OFDM). In OFDM, a wireless band is divided into a plurality of sub-bands and the information transmission is received by multiple antennas at the receiver end. The antenna signals from each of the antennas are converted to digital, then into the frequency domain, where selection of a signal for each sub-band occurs and then the sub-bands are combined and decoded.
Orthogonal Frequency Division Multiplexing (OFDM) has become popular for achieving high data rate and combating multipath fading in wireless communications. Currently, most standards for broadband wireless communications, such as IEEE 802.11a/g, ETSI-BRAN HIPERLAN/2 and DVB-T have adopted OFDM as the key technology. OFDM subdivides a carrier into several individually modulated orthogonal sub-carriers. In other words, the entire frequency selective fading channel can be decomposed into multiple flat fading ones to effectively mitigate the effects of delay spread and inter-symbol interference (ISI).
For example, in 802.11a, each channel is 20 MHz wide and is subdivided into 52 sub-carriers which are modulated using binary or quadrature phase shift keying (BPSP/QPSK), 16-quadrature amplitude modulation (QAM) or 64QAM, each spaced 312.5 kHz apart. Forty-eight of these sub-carriers are used for data, and the remaining four are pilot tones for error correction. The 802.11a standard specifies operations over 5.15-5.25, 5.25-5.35 and 5.725-5.825 GHz unlicensed national information structure (U-NII) bands. The 802.11a system provides data payload communication capabilities of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s.
To mitigate channel fading, multiple antennas in the receiver can be used to achieve spatial diversity. Multiple Receive Antenna combining techniques can basically be split into two groups: frequency domain combining and time domain combining. Frequency domain combining can increase the performance of the OFDM system by combining signals based on the sub-carrier information after a discrete Fourier transform (DFT) processor, whereas time domain combining does the same thing before DFT processor, which relaxes the hardware complexity.
In terms of the bit error rate (BER) performance, the sub-carrier based frequency domain combining technique is optimum. See, e.g., Butler et al., “The Performance of HIPERLAN/2 System with Multiple Antennas,” Vehicular Technology Conference, 2001. VTC 2001 Spring. IEEE VTS 53rd, Volume: 3, 6-9 (May 2001). However, the frequency domain combining technique uses multiple analog to digital (A/D) and discrete Fourier transform (DFT) processors, with an A/D and DFT for each receive antenna. Combining then occurs in the frequency domain. Therefore, frequency domain combining also can be referred to as post-DFT combining. Post-DFT combining is not a major problem in an access point (AP), but it is a big concern for a mobile terminal (MT), which benefits from lower hardware complexity and power demands. There are three types of well-known frequency domain combining techniques, selection diversity (SC), equal gain combining (EGC) and maximal ratio combining (MRC).
To reduce the hardware complexity of OFDM systems with multiple receive antennas, some time domain combining techniques have been proposed. In one technique with pre-DFT combining, the number of DFT processors can be dramatically reduced to one with some performance degradation. See, e.g., M. Okada, S. Komaki, “Pre-DFT Combining Space Diversity Assisted COFDM,” IEEE Trans. Vehicular Tech., vol. 50, No. 2, pp. 487-496 (March 2001). However that approach requires multiple A/Ds and is only applicable when the number of distinct paths in the channel are very limited. Another approach that has been proposed makes use of single analog front-end with multiple receive antennas and single baseband demodulator, but sacrifices data transfer rate and needs additional processing on the transmitter side. See, Jung et al., “A Subcarrier Selection Combining Technique for OFDM Systems,” IEICE Trans. Commun., Vol. E86-B, No. 7 (July 2003).
Hardware complexity can be dramatically reduced through simple antenna selection diversity as well. However, for frequency selective channels, there always exists the possibility that some sub-carriers of an OFDM symbol demodulated from the selected antenna may have lower amplitude than the corresponding sub-carriers of the other receive antennas. See, e.g., Jung et al., “A Subcarrier Selection Combining Technique for OFDM Systems,” IEICE Trans. Commun., Vol. E86-B, No. 7 (July 2003). As a result, antenna selection diversity has substantially degraded performance compared to the case of optimum sub-carrier based Maximal Ratio Combining (MRC) for the case of two antennas.